Head
Head[expr] gives the head of expr.
Examples: Head[f[x]] ; Head[a + b] ; Head[4] ; Head[x] . See The Mathematica Book: Section 2.1.1 and Section 2.1.1.
Further Examples
Head[expr] gives the head of expr. 
; Head[a + b] 
; Head[4] 
; Head[x] 
. 
